Electrostatic shielding films are used to form packages for a variety of electronic components. These films generally protect the component from electrostatic fields and voltages. As time has passed electronic components have become increasingly sensitive to electrostatic voltages and amperages. In addition, as tolerances for components become more exacting, changes in tolerances caused by corrosion become more important.
For example, the read/write heads of a disc drive fly over magnetically coated spinning disc and read the changes in the magnetic flux as data. As the head becomes more sensitive to changes in the magnetic flux, more data can be stored on the disc. Recently the disc drive industry began replacing inductive data reading heads with magnetoresistant (MR) and giant magnetoresistant (GMR) heads. These heads are many more times sensitive to magnetic flux than the prior heads. MR/GMR heads utilize thinner conductors and dielectrics as compared with inductive heads. This structure makes the heads far more sensitive to transient electrical current.
A common and unavoidable source of such transient electrical current is the human body. Static charge accumulates on the body through triboelectrification. The dynamic release of this stored charge through an electrostatic discharge can achieve impressive current and energy levels. Discharge from the body is simulated by a resistive/capacitive network with a resistance equal to 1500 ohms and a capacitance equal to 100 picofarads. Such an R/C network can produce voltages in excess of 15,000 volts. Testing of MR heads show changes in the resistance of the device with discharges of only 35 volts. More importantly, the magnetic failure threshold has also been shown to be 35 volts with current at 23 milliamps and energy at 0.9 nanojoules. Melting or pitting of the device layers occur at about 80 volts of discharge. Increased electrostatic protection of these devices is, therefore, increasingly important.
Corrosion of electronic components becomes increasingly important as the products become smaller and thinner. Corrosion is particularly important with respect to the MR heads. Because MR heads are more sensitive to magnetic flux they can be flown closer to the disc. Because the MR head can be flown closer to the disc, the space needed to store information is reduced. The space between the head and the disc may be less than 0.05 micrometers. As technology advances, this distance is decreasing. Because corrosion changes the dimensional tolerances between parts it becomes increasingly important to limit corrosion in electronic components which have close physical tolerances.
Most corrosion of electronic components is a function of the amount of moisture or water vapor the material is in contact with. Steps can be taken during manufacturing to exclude water vapor from the package by introducing dry air. However, during transportation, moisture vapor may penetrate the packaging.
Similarly measures are generally observed during the manufacture and mounting of MR/GR heads to avoid damage by transient currents from electrostatic discharge. All work surfaces are rendered free of static electricity, and special attention is focused on controlling the flow of charge from materials of dissimilar resistance. However, MR heads and similarly susceptible electronic components must be transported. For example, MR heads and assemblies containing MR heads are typically transported from the place where the head is manufactured to the place where the head is assembled, and then to the final drive assembly. During transport MR heads, and other electronic components are exposed to electrostatic discharge and water vapor. In order to provide a high level of protection, the material should shield the product from electrostatic charge, avoid charge retention, and prevent water vapor from penetrating the material.
Prior art products do not provide sufficient electrostatic and moisture protection. U.S. Pat. No. 4,699,830 issued to Mr. White on Oct. 13, 1987. Mr. White disclosed an inner layer of electrostatic material, a first metal layer, a single film layer, a second metal layer, and a thin clear abrasion resistant layer on the outer surface of the film material. This material was transparent. In order for the material to be clear the metal layers must be extremely thin, on the order of 20 Angstroms thick each. Metallic materials of this thickness are unable to provide sufficient protection against external voltages in excess of approximately 1,000 volts. In addition, metals sufficiently thin to be transparent will not provide a moisture barrier. Finally, the practical way to achieve metal layers sufficiently thin to be transparent is to vapor deposit such metals onto a plastic film. Vapor deposition on two surfaces of a single film layer as disclosed in White is difficult and expensive.
Another patent which attempts to address the issue of providing a moisture barrier and some electrostatic protection is U.S. Pat. No. 4,971,196 issued to Kitamura on Nov. 20, 1990. This patent discloses three embodiments, one of which is particularly relevant. This embodiment has a structure with an inner layer of polyethylene with antistatic attached to an aluminum foil layer which in turn is attached to a polyester film layer, and then attached to a conductive carbon loaded plastic layer, and finally a non-conductive acrylic type protective layer over the carbon conductive layer. This material suffers from several problems. While the aluminum foil and polymeric layer will provide a limited moisture barrier and electrostatic shielding, the outer polymer layers will retain charge. This charge retention may lead to electrostatic discharges which will damage the component inside the packaging when the component is removed from the package. Furthermore, because the moisture and electrostatic protection is in a single layer, any punctures, pinholes, and other small defects in this layer will compromise the protection. In addition, metal foils are more expensive than metallized polymers, and are more difficult to handle.